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501.105(B)(1) and B(3)

I am trying to determine if a general purpose Instrument is acceptable for a Class I Div 2 hazardous area. A lot of vendors offer Intrinsically Safe or Explosion Proof variants, but this protection method is primaraly for Class I Div 1, and not necessary for Div 2. It seems to me that if the non-classified standard instrument meets 501.105, and is not included in 501.115-501.150, then it is acceptable in Class I Div 2.

I am looking at NFPA 70 2017 501.105 Meters, Instruments, Relays.

For 501.105(B)(1), does the "general purpose enclosure" refer to the overall enclosure that the device is mounted in: like a hoffman box? Or, does it refer to the actual device casing?

For 501.105(B)(3), do not all electronic Instruments contain some kind of resistor on their PCB's? Resistors are a basic building block of electronics, and I would expect to find at least one on any electronic instrument...

I am trying to determine if a general purpose Instrument is acceptable for a Class I Div 2 hazardous area.

Knowing what I know about equipment standards, I'd be skeptical that comparison against the NEC would be sufficient.

To determine suitability on anything more complicated than a junction/relay box would usually mean a complete field evaluation to ISA 12.12.01, or some similar equipment standard. Which is why field inspectors exist.

It is not unusual to find instruments that are listed as suitable for CID2. I think this just means it is non-incendive which presumably means that someone reliable like UL has evaluated it and determined that among other things the surface temperature of the devices contained within the instrument will not cause ignition. You might want to look for such a device. These days it is not uncommon.

ISA 12.12.01 looks like a very interesting standard. Is the intent of ISA 12.12.01 to be used on an overall machine assembly? For example, if I have an existing machine in the field, and want to have it inspected, a field inspector would use ISA 12.12.01 to inspect the equipment to verify its suitability for a hazardous location?

This is different than the approach I have seen in the past, where you design looking for each individual component one at a time. It seems like ISA 12.12.01 would provide a way to inspect equipment as a whole, regardless of the markings etc., of the individual components. Is that correct?

ISA 12.12.01 looks like a very interesting standard. Is the intent of ISA 12.12.01 to be used on an overall machine assembly? For example, if I have an existing machine in the field, and want to have it inspected, a field inspector would use ISA 12.12.01 to inspect the equipment to verify its suitability for a hazardous location?

This is different than the approach I have seen in the past, where you design looking for each individual component one at a time. It seems like ISA 12.12.01 would provide a way to inspect equipment as a whole, regardless of the markings etc., of the individual components. Is that correct?

ANSI/ISA 12.12.01 can certainly be used on a complete apparatus. Or on components - it's written to accommodate both.

A lot of equipment certified for use in Division 2 has been evaluated to 12.12.01 as a unit, not as a collection of components. This can be done via inspection at an agency, or by a field inspection.

However, to do a field inspection, it is typically necessary to have the design drawings for the equipment available. This is because, in many (if not most) cases, compliance cannot be verified by simply looking at the equipment. There may be aspects of the design which are not visible or inaccessible.

In rare cases, it is theoretically possible to do all of the evaluation to 12.12.01 without having the design drawings available. But that is a risky way to go, in my experience.